1. Field of the Invention
The present invention relates to an overcurrent protection device for a Class-D amplifier, particularly to a voltage detection type overcurrent protection device for a Class-D amplifier, which applies to a two-state output (1 or 0) high-power conversion efficiency Class-D amplifier or a general PWM system.
2. Description of the Related Art
Refer to FIG. 1. Generally, a Class-D amplifier adopts a current sensor to detect current, wherein a current mirror 12 copies an output current to another circuit by a very small proportion, and detects whether the copied current exceeds a threshold. Such a design usually at least needs an operational amplifier 16 and a current comparator 14. Therefore, when realized with an integrated circuit, the conventional current detection circuit is more complicated, less power-efficient and less space-efficient.
Refer to FIG. 2 for a conventional CMOS (Complementary Metal Oxide Semiconductor) Class-D amplifier. An audio signal is amplified by an audio amplifier 18 and then output to a comparator 20. The comparator 20 compares the amplified audio signal with a signal of a triangle generator 22 and outputs the result to a clock logic module 24. CMOS transistors 26 succeed to the clock logic module 24 and function as the output stage. The output terminals of the CMOS transistors 26 are connected to a filter circuit containing inductors 28 and capacitors 30, and the filter circuit is then connected to a loudspeaker 32.
A Taiwan patent No. 560125 disclosed a “Current Detection and Overcurrent Protection Circuit for Transistors of PWM Amplifier”. The Claim 4 of the prior-art patent disclosed an overcurrent protection circuit for a switching circuit, which uses at least one switching transistor P1 to provide a given load current IL for a load and which comprises: a sampling/holding capacitor C2, a switch 31, an overcurrent detection device 50, and a controller 60. The sampling/holding capacitor C2 is used to temporarily hold a terminal voltage VP1 of the turned on switching transistor P1. The switch 31 is arranged in between the switching transistor P1 and the sampling/holding capacitor C2, and is turned on synchronously with the switching transistor P1. The overcurrent detection device 50 is used to determine whether a detected voltage VS1, which is corresponding to a voltage charging the sampling/holding capacitor C2, exceeds a predetermined reference voltage VREF. When the detected voltage VS1 is over the reference voltage VREF, the controller compulsorily turns off the switching transistor P1. From the above description, it is known that the prior-art overcurrent protection circuit uses the sampling/holding capacitor to detect overcurrent.
Contrasting to the prior art, the present invention proposes a voltage detection type overcurrent protection device to detect overcurrent.